Late goals are woven into the fabric of Manchester United, but there are times when the champions of Europe can become unstitched. Two days after Federico Macheda had restored United's reputation for delivering late drama, Carlos Tevez appeared to have replicated the teenage Italian's trick by emerging from the bench to secure a lead.

While Old Trafford savoured United's indefatigability, however, Porto acquainted the defending champions with the feeling of dejection a side denied at the death suffers.

It was both belated and deserved when Mariano became the second substitute to score, excelling in defeating Edwin van der Sar after Lucho Gonzalez's cross glanced off the head of John O'Shea and thus completing a chastening evening for the Irishman.

"We have to think the last goal was one we could not afford," admitted Sir Alex Ferguson. "It was a terrible goal to lose. We have only ourselves to blame." It leaves United effectively needing to win at the Dragao Stadium next Wednesday if they are to reach a third successive semi-final. "It's a difficult job now," added Ferguson. "It's not made any better when they tell you no British team has won in Porto."

Now they require an upset. Having spent much of the season in a relentless campaign based on subduing and conquering as quietly as possible, Manchester United have now provided a reminder of their capacity to shock. That, however, was not something to savour.

There were times when they were shocking, especially defensively. Their 14 successive Premier League clean sheets are now banished to the recesses of memory; once experts at 1-0 victories, they have now sieved 10 goals in four games. But for a string of saves from van der Sar, Porto would have mustered more than two on Tuesday night.

Their manager Jesualdo Ferreira said: "I think my players deserved what they did on the pitch. We tried to pose difficulties to United. It's a good score but nothing is sorted yet. Manchester United had more possession but we had more opportunities."

Arsene Wenger may have called Arsenal the "super-outsiders", but many had felt Porto merited that description. Ferguson, who had praised the Portuguese league leaders before the game, was not among them but his charges did seem to subscribe to that view.

"I'm not a bit surprised," said the United manager. "That's exactly the way they've been playing away from home. Their game against Atletico [Madrid] was exactly the same. The front three were a handful."

They were, and Ferguson's was an accurate assessment of United's failings. "What didn't help us today was the start to the match was so positive by Porto," he added. "We got nervous. We never got the rhythm of our game going for half an hour."

In contrast, Porto prospered. Cristian Rodriguez steered in a fourth-minute goal that was a reward for their enterprise. Both the Uruguayan and Lisandro Lopez kept van der Sar occupied with a series of skilful raids. The return of Nemanja Vidic did not end United's defensive jitters.

Rodriguez was adept at finding space between Jonny Evans and Patrice Evra while O'Shea proved as error-prone as Gary Neville had been two days before. With the midfield trio affording insufficient protection, Ferguson made a rare first-half reshuffle; 4-3-3 became 4-4-1-1.

It was remedy, though not a cure. Yet while United were wasteful in possession, the most costly misplaced pass came from Bruno Alves, who sent Wayne Rooney through to equalise. The Englishman's deft finish was as well judged as the Portuguese's pass was misjudged.

Indeed, Rooney was the honourable exception, providing energy where others offered lethargy. It was fitting that he supplied United's second goal, a beautiful volleyed backheel flicking Neville's throw into the path of Tevez, who finished at the near post. "It's a typical Carlos Tevez goal," added Ferguson. "He's very quick in that area of the pitch." It was indicative, too, that the move involved two late additions to proceedings.

It was the second successive match where Ferguson's replacements have produced a significant improvement, which can prompt praise for the manager or questions about his choices. With Ryan Giggs displaying his enduring class on his 797th game, Neville finally offering an attacking outlet at full-back and Tevez increasing the tempo in a frenzied cameo, United's second-half showing was more recognisable.

The denouement, however, was not. Enthralled by a late goal 52 hours before, they may have been eliminated by one now.

MAN OF THE MATCH: Wayne Rooney - Unflagging in his efforts and capable of moments of high quality, Rooney maintained his excellence while others floundered.

MANCHESTER UNITED VERDICT: Fatigue may have played a part in a subdued showing, but there was cause for concern in the first-half display. Ji-sung Park's lack of incision or invention was apparent and, for once, he was "well off the pace" in his manager's words. Ferguson was also correct to curtail Paul Scholes' contribution. The veteran may be deemed a European specialist, given the slower speed of the continental game, but he was unable to get close enough to Porto's sprightly trio. It may not bode well for Scholes that they fared better with a central duo of Darren Fletcher and Michael Carrick after his exit.

PORTO VERDICT: Technically adept and skilful, they posed considerable problems. The impressive, if not incredible, Hulk formed part of an interchangeable front trio with Rodriguez and Lopez. Gonzalez was terrific in midfield.

EVANS ABOVE: Jonny Evans joined Manchester United's lengthy injury list with a groin problem. With Rio Ferdinand sidelined, it is possible they will go to Sunderland on Saturday with O'Shea, who was abject, accompanying Vidic in the centre of defence.